Steel sheet hot-dip plated with zinc based layer with superior bake hardenability and aging resistance, and manufacturing method thereof

ABSTRACT

Provided are a cold-rolled steel sheet having excellent bake hardenability and aging resistance, and manufacturing method thereof. The cold-rolled steel sheet comprises, by weight, 0.02 to 0.08% of carbon (C), 1.3 to 2.1% of manganese (Mn), 0.3% or less (excluding 0%) of silicon (Si), 1.0% or less (excluding 0%) of chromium (Cr), 0.1% or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur (S), 0.01% or less (excluding 0%) of nitrogen (N), and 0.01 to 0.06% of acid soluble aluminum (sol.Al), comprises one or more selected from the group consisting of 0.2% or less (excluding 0%) of molybdenum (Mo) and 0.003% or less (excluding 0%) of boron (B), and comprises a remainder of iron (Fe) and unavoidable impurities, and comprises, by area, 90 to 99% of ferrite and 1 to 10% of martensite as a microstructure.

TECHNICAL FIELD

The present disclosure relates to a steel sheet hot-dip plated with zincbased layer, having excellent bake hardenability and aging resistance,and a manufacturing method thereof, and more particularly, to a steelsheet hot dip plated with zinc based layer, having excellent bakehardenability and aging resistance, preferably capable of being used asa material for external automobile panels, and a manufacturing methodthereof.

BACKGROUND ART

As impact stability regulations of automobiles and fuel efficiency areemphasized, high tensile steel has been actively used to satisfyrequirements for both weight reductions and high strength in automobilebodies. In accordance with this trend, the application of high-strengthsteel to external automobile panels has also been extended.

Currently, most 340 MPa-grade bake hardened steel has been used asexternal automobile panels, but a portion of 490 MPa-grade steel sheetsare also being applied, which will be expected to be extended to 590MPa-grade steel sheets.

As described above, when such steel sheets having increased strength areapplied as an external panel, weight reduction and dent resistance maybe improved. On the other hand, as strength increases, there is adisadvantage that formability may be deteriorated. Accordingly,recently, customers are demanding a steel sheet having a relatively lowyield ratio (YR=YS/TS) and relatively high ductility, in order tosupplement poor workability while high-strength steel may be applied touse in an external panel.

In addition, it is necessary to have bake hardenability at a certainlevel or higher in order for a material to be applied to use in externalautomobile panels. A phenomenon of bake hardenability is a phenomenon inwhich yield strength is increased due to fixing solid solution carbonand nitrogen, which are activated during the press, onto dislocations atthe time of the baking of paint. Steel having excellent bakehardenability is easy to form before the baking of paint, and finalproducts thereof have enhanced dent resistance. Therefore, such steel isvery ideal as a material for external automobile panels. In addition, inorder to be a material applied to use in external automobile panels, itis necessary to have a certain level of aging resistance to guaranteeaging for a certain period or longer.

Japanese Patent Publication No. 2005-264176 discloses a steel sheethaving a complex phase mainly composed of martensite as a conventionaltechnique for improving workability in a high-strength steel sheet. Inorder to improve workability, a method of manufacturing a high-strengthsteel sheet in which a fine Cu precipitate has a grain size of 1 to 100nm is disclosed. However, in this technique, it is necessary to addexcessive amounts of Cu of 2 to 5% in order to precipitate fine Cuparticles. In this case, hot shortness attributable to Cu may occur, andmanufacturing costs may be excessively increased.

Japanese Patent Publication No. 2004-292891 discloses a steel sheethaving a complex phase including ferrite as a main phase and residualaustenite and bainite and martensite which are low temperaturetransformation phases as secondary phases, and a method for improvingductility and stretch flangeability of the steel sheet. However, thistechnique has problems in that it may be difficult to secure platingquality, and to secure surface quality in a process for making steel anda continuous casting process, since large amounts of Si and Al are addedto secure the residual austenite phase. In addition, there is adisadvantage in that yield ratio may be high because an initial YS valueis high due to transformation induced plasticity.

Korean Patent Publication No. 10-2002-0073564 discloses a technique forproviding a high tensile hot-dip galvanized steel sheet having goodworkability. A steel sheet comprising soft ferrite and hard martensiteas a microstructure, and a manufacturing method for improving anelongation and an r value (a Lankford value) of the steel sheet aredisclosed. However, this technology has a problem that it is difficultto secure good plating quality, since large amounts of Si are added, anda problem that manufacturing costs increase due to the addition of largeamounts of Ti and Mo.

DISCLOSURE Technical Problem

One of the objects of the present disclosure is to provide a steel sheethot-dip plated with zinc based layer, having excellent bakehardenability and aging resistance, and a manufacturing method thereof.

Technical Solution

According to an aspect of the present disclosure, a steel sheet hot-dipplated with zinc based layer, having excellent bake hardenability andaging resistance, comprises a cold-rolled steel sheet and a zinc basedplating layer formed on a surface of the cold-rolled steel sheet,wherein the cold-rolled steel sheet comprises, by weight, 0.02 to 0.08%of carbon (C), 1.3 to 2.1% of manganese (Mn), 0.3% or less (excluding0%) of silicon (Si) , 1.0% or less (excluding 0%) of chromium (Cr), 0.1%or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%)of sulfur (S), 0.01% or less (excluding 0%) of nitrogen (N), and 0.01 to0.06% of acid soluble aluminum (sol.Al), comprises one or more selectedfrom the group consisting of 0.2% or less (excluding 0%) of molybdenum(Mo) and 0.003% or less (excluding 0%) of boron (B), and comprises aremainder of iron (Fe) and unavoidable impurities, and comprises, byarea, 90 to 99% of ferrite and 1 to 10% of martensite as amicrostructure, wherein a ratio (a/b) of an average carbon concentrationa in the martensite and an average carbon concentration b in the ferritelocated in a virtual circle having a diameter corresponding to a longaxis of the martensite at the point of ¼ t of a sheet thickness of thecold-rolled steel sheet is 1.4 or less, and wherein a ratio (d/c) of anaverage manganese concentration c in the martensite and an averagemanganese concentration d in the ferrite located in a virtual circlehaving a diameter corresponding to a long axis of the martensite at thepoint of ¼ t of a sheet thickness of the cold-rolled steel sheet is 0.9or less.

According to another aspect of the present disclosure, a method ofmanufacturing a steel sheet hot-dip plated with zinc based layer, havingexcellent bake hardenability and aging resistance, comprises reheating asteel slab comprising, by weight, 0.02 to 0.08% of carbon (C), 1.3 to2.1% of manganese (Mn) , 0.3% or less (excluding 0%) of silicon (Si) ,1.0% or less (excluding 0%) of chromium (Cr), 0.1% or less (excluding0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur (S), 0.01%or less (excluding 0%) of nitrogen (N), and 0.01 to 0.06% of acidsoluble aluminum (sol.Al), comprising one or more selected from thegroup consisting of 0.2% or less (excluding 0%) of molybdenum (Mo) and0.003% or less (excluding 0%) of boron (B), and comprising a remainderof iron (Fe) and unavoidable impurities;

hot-rolling the reheated steel slab in a single phase temperature regionof austenite to obtain a hot-rolled steel sheet; coiling the hot-rolledsteel sheet; cold-rolling the coiled hot-rolled steel sheet to obtain acold-rolled steel sheet; continuously annealing the cold-rolled steelsheet at a temperature in a range of 760 to 850° C.; firstly cooling thecontinuously annealed cold-rolled steel sheet to a temperature in arange of 630 to 670° C. at an average cooling rate of 2 to 14° C./sec;secondly cooling the firstly cooled cold-rolled steel sheet to atemperature in a range of (Ms+20)−(Ms+50)° C. at an average cooling rateof 3 to 12° C./sec; thirdly cooling the secondly cold-rolled steel sheetto a temperature in a range of 440 to 480° C. at a rate of 4 to 8°C./sec; immersing the thirdly cooled cold-rolled steel sheet in a zincbased hot bath to obtain a steel sheet hot-dip plated with zinc basedlayer; and finally cooling the steel sheet hot-dip plated with zincbased layer to a temperature in a range of (Ms−100)° C. or lower at anaverage cooling rate of 3° C./sec or higher.

Advantageous Effects

As one of various effects of the present disclosure, the galvanizedsteel sheet according to an embodiment of the present disclosure may besuitably applied to a material for external automobile panels, becauseof its excellent bake hardenability and aging resistance.

BEST MODE FOR INVENTION

Hereinafter, exemplary embodiments of the present disclosure will bedescribed in detail.

The inventors of the present disclosure have conducted intensiveresearch into providing a steel sheet hot-dip plated with zinc basedlayer securing excellent strength and ductility simultaneously to haveexcellent formability, as well as excellent bake hardenability and agingresistance, so as to be suitable as a material for external automobilepanels. As a result, it became possible to provide a steel sheet hot-dipplated with zinc based layer which satisfies the intended properties byoptimally controlling a composition range of a cold-rolled steel sheet,a substrate, and optimizing production conditions thereof. Finally, thepresent disclosure has been accomplished based on this finding.

Hereinafter, a steel sheet hot-dip plated with zinc based layer, havingexcellent bake hardenability and aging resistance, an aspect of thepresent disclosure, will be described in detail.

The steel sheet hot-dip plated with zinc based layer of the presentdisclosure may include a cold-rolled steel sheet and a zinc basedhot-dip plating layer formed on one or both surfaces of the cold-rolledsteel sheet. In the present disclosure, a composition of the zinc basedhot-dip plating layer is not particularly limited, and may be a pureZinc plating layer, or a Zinc based alloy plating layer containing Si,Al, Mg, or the like. The zinc based hot-dip plating layer may beagalva-annealed layer.

Hereinafter, the alloying element and the preferable content rangethereof of the cold-rolled steel sheet as a substrate will be describedin detail. It is to be noted in advance that the content of eachcomponent described below is on a weight basis unless otherwisespecified.

-   -   Carbon (C): 0.02 to 0.08%

Carbon may be an indispensable element to be added to secure the desiredcomplex phase in the present disclosure. Generally, carbon isadvantageous for producing a complex phase since martensite may beeasily formed as the content of carbon increases. However, to secure theintended strength and yield ratio (yield strength/tensile strength), itis necessary to control the content in a proper amount. When the contentof carbon is less than 0.02%, it may be difficult to achieve the desiredstrength in the present disclosure, and formation of an appropriatelevel of martensite may be difficult. On the other hand, when thecontent thereof exceeds 0.08%, the formation of bainite at the grainboundary may be promoted during cooling after annealing to increase theyield ratio of the steel, and bending and surface defects may be easilycaused in machining into automobile parts. Therefore, in the presentdisclosure, the content of carbon may be controlled to be 0.02 to 0.08%,and more preferably 0.03 to 0.06%.

-   -   Manganese (Mn): 1.3 to 2.1%

Manganese may be an element which improves the hardenability in thecomplex phase steel, and, in particular, plays an important role informing martensite. When the content of manganese is less than 1.3%, theformation of martensite may be impossible, and complex phase steel maybe difficult to be produced. On the other hand, when the content ofmanganese exceeds 2.1%, martensite may be excessively formed to make amaterial property unstable, and there may be a problem that the risk ofprocessing crack and strip breakage is significantly increased due tothe formation of a band of manganese in the structure. In addition,there may be a problem that the manganese oxide is precipitated on thesurface upon annealing, which significantly deteriorates platingcharacteristics. Therefore, in the present disclosure, the content ofmanganese may be controlled to be 1.3 to 2.1%, and more preferably to1.4 to 1.8%.

-   -   Silicon (Si): 0.3% or less (excluding 0%)

Silicon may contribute to an increase in the strength of the steel sheetby solid solution strengthening, but may be not intentionally added inthe present disclosure. Further, there may be no problem in securing theproperties without adding silicon. However, 0% may be excluded inconsideration of an amount that is inevitably added in the manufacturingprocess. On the other hand, when the content of silicon exceeds 0.3%,there may be a problem that the surface properties of the plating may bepoor. Therefore, the content of silicon may be controlled to be 0.3% orless in the present disclosure.

-   -   Chromium (Cr): 1.0% or less (excluding 0%)

Chromium may be a component having characteristics similar to manganese,and may be an element added to improve hardenability of steel, and toimprove strength of steel. In addition, chromium may assist in formingmartensite. Further, since an occurrence of yield stretch YP-El issuppressed by precipitating solid solute carbon to be under a certainlevel which is proper amount of solute carbon in the steel throughforming coarse Cr-based carbides such as Cr23C6 during hot-rolling,chromium may be an element favorable for the production of complex phasesteel having a relatively low yield ratio. In addition, chromium is anelement advantageous for manufacturing high strength complex phase steelhaving a relatively high ductility by relatively reducing ductility dropcompared with the increase in strength. However, when the contentthereof exceeds 1.0%, the martensite structure fraction may beexcessively increased to cause a decrease in strength and elongation. Inthe present disclosure, the content of chromium may be controlled to be1.0% or less.

-   -   Phosphorus (P): 0.1% or less (excluding 0%)

Phosphorus is the most advantageous element in securing strength withoutsignificantly impairing formability. However, the possibility of theoccurrence of brittle fracture significantly increases when the elementis excessively added, the possibility of strip breakage of a slabsignificantly increases during hot-rolling, and the surface propertiesof a plated layer may be deteriorated. Therefore, in the presentdisclosure, the content of phosphorus may be controlled to be 0.1%.

-   -   Sulfur (S): 0.01% or less (excluding 0%)

Sulfur may be an impurity to be inevitably contained in the steel. Itmay be desirable to control the content of sulfur to be as low aspossible. In particular, sulfur in the steel may increase thepossibility of generating hot shortness, and the content thereof may becontrolled to be 0.01% or less.

Nitrogen (N): 0.01% or less (excluding 0%)

Nitrogen may be an impurity to be inevitably contained in the steel. Itmay be desirable to control the content of nitrogen as low as possible.However, since the steel refining cost rises sharply to reduce thecontent of nitrogen, the content thereof may be controlled to be 0.01%or less, a possible range of operation conditions.

-   -   Acid soluble aluminum (sol.Al): 0.01 to 0.06%

Acid soluble aluminum is an element to be added for grain refinement anddeoxidation. When the content thereof is less than 0.01%,aluminum-killed (Al-killed) steel may not be produced in a normal stablestate. Meanwhile, when the content thereof exceeds 0.06%, it isadvantageous to increase the strength due to the grain refinementeffect. On the other hand, when the steelmaking operation in acontinuous casting process is carried out, the inclusions may beexcessively formed. In this case, the possibility of surface defects ofa plated steel sheet may increase, and a sharp rise in manufacturingcosts may occur. Therefore, in the present disclosure, the content ofacid soluble aluminum may be controlled to be 0.01 to 0.06%.

One or more selected from the group consisting of 0.2% or less(excluding 0%) of molybdenum (Mo) and 0.003% or less (excluding 0%) ofboron (B)

Molybdenum may be an element added to delay transformation of austeniteinto pearlite, and to improve ferrite refinement and steel strength.Molybdenum may also assist in improving hardenability of steel. However,when the content of molybdenum exceeds 0.1%, there may be a problem inthat manufacturing costs are rapidly increased to lower economicalefficiency and to lower ductility of steel. In the present disclosure,the content of molybdenum may be controlled to be 0.1% or less.

In addition, boron may be an element added to prevent secondary workembrittlement caused by phosphorous in the steel. There may be noproblem in securing the properties without adding boron. Meanwhile, whenthe content of boron exceeds 0.003%, there may be a problem thatductility of the steel is lowered. In the present disclosure, thecontent of boron may be controlled to be 0.003% or less.

In addition, iron (Fe) and unavoidable impurities may be furtherincluded as a remainder. However, in the ordinary manufacturing process,impurities that are not intended from raw materials or surroundingenvironments may be inevitably incorporated, such that it may not beexcluded. Such impurities are not specifically mentioned in thisspecification, as they are known to one of ordinary skill in the art. Inaddition, the addition of an effective component other than theabove-mentioned composition may be not excluded.

The cold-rolled steel sheet of the present disclosure may include, byarea, 90 to 99% of ferrite and 1 to 10% of martensite as amicrostructure.

When an area ratio of the martensite is less than 1%, it may bedifficult to form a complex phase and it may be difficult to obtain asteel sheet having a relatively low yield ratio. On the other hand, whenthe area ratio exceeds 10%, the strength may be excessively increased.Therefore, an area ratio of martensite is preferably 1 to 10%, morepreferably 2 to 5%, by area.

In the cold-rolled steel sheet of the present disclosure, a ratio (a/b)of an average carbon concentration a in the martensite and an averagecarbon concentration b in the ferrite located in a virtual circle havinga diameter corresponding to a long axis of the martensite at the pointof ¼ t of a sheet thickness thereof may be a value of 1.4 or less.

In the present disclosure, fine martensite in a ferrite matrix may beappropriately distributed. At the same time, a ratio of the carbonconcentration in an interior of martensite and in an interior of ferritein a periphery of the martensite may be appropriately controlled. Inaccordance therewith, it may be designed such that the carbonintensively present in martensite can easily diffuse into surroundingferrite by the conventional baking treatment (about 170° C., about 20minute). When the ratio (a/b) of the average carbon concentrationexceeds 1.4, the content of the solid solution carbon present in ferriteis too low to secure the desired bake hardenability. Meanwhile, as theratio (a/b) of the average carbon concentration lowers, the securing ofbake hardenability may be relatively high. Therefore, the lower limit isnot particularly limited in the present disclosure.

In the cold-rolled steel sheet of the present disclosure, a ratio (d/c)of an average manganese concentration c in the martensite and an averagemanganese concentration d in the ferrite located in a virtual circlehaving a diameter corresponding to a long axis of the martensite at thepoint of ¼ t of a sheet thickness thereof may be a value of 0.9 or less,more preferably a value of 0.8 or less. When the ratio (d/c) of theaverage manganese concentration exceeds 0.9, the content of manganesepresent in ferrite is too high to facilitate the formation of amanganese band in the structure. The possibility of processing cracks informing may increase due to the decrease in ductility of steel.Meanwhile, as the ratio (d/c) of the average manganese concentrationlowers, the securing of ductility may be relatively high. Therefore thelower limit is not particularly limited in the present disclosure.

According to an embodiment, an occupancy ratio (M) of martensite havingan average circle equivalent diameter of 5 μm or less (excluding 0 μm)present at ferrite grain boundaries (including grain boundary triplepoints) defined by the following Relationship 1 may be 90% or more:

[Relationship 1] M ={M _(gb)/(M _(gb)+M _(in))}×100

where M_(gb) refers to the number of martensite having an average circleequivalent diameter of 5 μm or less (excluding 0 μm) present at ferritegrain boundaries, and M_(in) refers to the number of martensite havingan average circle equivalent diameter of 5 μm or less (excluding 0 μm)present inside ferrite crystal grains.

That is, as the fine martensite having an average circle equivalentdiameter of 5 μm or less (excluding 0 μm) is mainly present at ferritegrain boundaries rather than inside ferrite crystal grains, it may beadvantageous in improving ductility with maintaining a relatively lowyield ratio. When the occupancy ratio (M) of martensite is less than90%, martensite formed in the crystal grains may increase yield strengthduring tensile deformation to increase yield ratio. In this case, it maybe difficult to control the yield ratio through temper rolling. Inaddition, martensite existing in the crystal grains may significantlyinhibit a moving of dislocation during processing and weaken ductilityof ferrite, such that a reduction of elongation may be caused.

In the meantime, the cold-rolled steel sheet of the present disclosuremay partially contain bainite in addition to the above-mentioned ferriteand martensite. Since solid solute carbon and solid solute nitrogenexisting inside the grains of bainite may easily adhere to adislocation, interfere with the displacement of the dislocation, andexhibit a discontinuous yield behavior to remarkably increase a yieldratio of steel. Therefore, in the present disclosure, the formation ofbainite is preferred to be inhibited as much as possible.

According to an embodiment, an area ratio (B) of the bainite defined bythe following Relationship 2 may be 3 or less. When the area ratio (B)of the bainite exceeds 3, the carbon concentration around the bainitemay increase to deteriorate ductility of steel, and a yield ratio mayrise sharply:

[Relationship 2] B={A _(B)/(A _(F)+A _(M)+A _(B))}×100

where A_(F) refers to an area ratio of ferrite, A_(M) refers to an arearatio of martensite, and A_(B) refers to an area ratio of bainite.

According to an embodiment, a plated layer may be formed on a surface ofthe cold-rolled steel sheet of the present disclosure. Such a platedlayer may be any one of a hot-dip galvanized layer or a galva-annealedlayer. As described above, when a cold-rolled steel sheet is formed withthe plated layer on its surface, corrosion resistance may be remarkablyimproved.

The steel sheet hot-dip plated with zinc based layer of the presentdisclosure described above may be produced by various methods, and theproduction method thereof is not particularly limited. As a preferableexample, it may be produced by the following methods.

Hereinafter, a method of producing a steel sheet hot-dip plated withzinc based layer, having excellent bake hardenability and agingresistance, another aspect of the present disclosure, will be describedin detail.

First, a steel slab having the above-mentioned component system may bereheated. This operation may be carried out to smoothly perform thesubsequent hot-rolling operation, and to sufficiently obtain thetargeted properties of the steel sheet. In the present disclosure,process conditions of the reheating operation are not particularlylimited, and may be normal conditions. As an example, a reheatingoperation may be performed in a temperature range of 1100 to 1300° C.

Next, the reheated steel slab may be hot-rolled in a single phasetemperature region of austenite to obtain a hot-rolled steel sheet. Thereason why a hot-rolling operation is carried out in the single phasetemperature region of austenite may be to increase the uniformity of thestructure.

According to an embodiment, during hot-rolling, a finish rollingtemperature may be within a range of (Ar3+50) to 950° C. When the finishrolling temperature is lower than (Ar3+50)° C., ferrite and austenitetwo-phase region rolling is highly likely to cause non-uniformity ofmaterial. On the other hand, when the temperature exceeds 950° C.,non-uniformity of material due to coarse grain caused byhigh-temperature rolling may occur, and a coil twisting phenomenon mayoccur during cooling of the hot-rolled steel sheet. For reference, atheoretical temperature of an Ar3 point may be obtained by the followingRelationship 3:

[Relationship 3] Ar3(° C.)=910−310[C]−80[Mn]−20[Cu]−15[Cr]−55[Ni]−80[Mo]

where each of [C], [Mn], [Cu], [Cr]. [Ni] and [Mo] refers to weight % ofthe respective elements.

Next, the hot-rolled steel sheet may be coiled.

According to an embodiment, the coiling temperature may be within arange of 450 to 700° C. When the coiling temperature is lower than 450°C., excess formation of martensite or bainite may lead to an excessiveincrease in strength of the hot-rolled steel sheet, which may causeproblems such as poor shape, and the like, due to the subsequent loadduring cold-rolling. On the other hand, when the coiling temperatureexceeds 700° C., surface enrichment of elements which lower wettabilityof hot-dip galvanized steel such as Si, Mn, B, and the like in the steelmay be significantly increased.

Next, the rolled hot-rolled steel sheet may be cold-rolled to obtain acold-rolled steel sheet.

According to an embodiment, in the cold-rolling operation, acold-rolling reduction ratio in the cold-rolling operation may be 40 to80%. When the cold-rolling reduction ratio is less than 40%, it may bedifficult to secure the target thickness, and it may be also difficultto correct a shape of the steel sheet. On the other hand, when thecold-rolling reduction ratio exceeds 80%, cracks may occur at an edgeportion of the steel sheet, and a cold-rolling load may be caused.

Next, the cold-rolled steel sheet may be continuously annealed. Thisoperation may be performed to form ferrite and austenite simultaneouslywith recrystallization, and to distribute carbon therein.

At this time, an annealing temperature may preferably be within a rangeof 760 to 850° C. When the annealing temperature is lower than 760° C.,sufficient recrystallization may be not achieved, and sufficientformation of austenite may be difficult, which make it difficult tosecure the desired strength in the present disclosure. On the otherhand, when the temperature exceeds 850° C., the productivity may belowered, austenite may be excessively formed, bainite may be formed inthe subsequent cooling operation, and ductility of steel may bedeteriorated.

Meanwhile, the above annealing temperature range may correspond to atwo-phase region (ferrite+austenite) temperature range, but annealing ispreferably carried out at a temperature range containing as much ferriteas possible. This is why as initial ferrite at the annealing temperatureof the two-phase region is relatively more, a growth of crystal grainafter annealing may be promoted to enhance ductility. Further, a degreeof carbon enrichment in austenite may be increased to lower amartensitic transformation starting temperature (Ms). In this case, itis possible to form martensite upon cooling after plating process, thesubsequent operation. In accordance therewith, it is possible to producea steel sheet having a relatively low yield ratio and a relatively highductility, since fine and uniform martensite is distributed in crystalgrains as much as possible. In consideration of this, the annealingtemperature may more preferably be within a range of 770 to 810° C.

Next, the cold-rolled steel sheet subjected to the continuouslyannealing operation may be firstly cooled in a temperature range of 630to 670° C. at an average cooling rate of 2 to 14° C./sec. In the presentdisclosure, as the firstly cooling end temperature is controlled to berelatively high, or the firstly cooling rate is controlled to berelatively slow, tendency of uniformity and coarsening of ferrite may beenhanced, advantageous for ensuring ductility of steel. In addition, inthe present disclosure, a sufficient time may be provided to allowcarbon to diffuse into austenite during the firstly cooling operation,which is significant in the present disclosure. More specifically, inthe two-phase temperature region, carbon may diffuse into austenitehaving a high degree of carbon enrichment. As the temperature thereof isrelatively high, a degree of the diffusion may increase. When thefirstly cooling end temperature is lower than 630° C., such anexcessively low temperature may result in a relatively low carbondiffusion activity. In this case, carbon concentration in ferrite mayincrease to result in an increase in yield ratio and an increase in atendency toward cracking during processing. On the other hand, when thefirstly cooling end temperature exceeds 670° C., it may be advantageousin terms of diffusion of carbon, but require an excessively high coolingrate in a secondly cooling operation of the subsequent process. When thefirstly cooling rate is lower than 2° C./sec, it may be disadvantageousin terms of productivity. On the other hand, when the firstly coolingrate exceeds 14° C./sec, diffusion of carbon may not sufficiently occur,thereby being not preferred.

Next, the firstly cooled cold-rolled steel sheet may be secondly cooledto a temperature in a range of (Ms+20) to (Ms+50)° C. at an averagecooling rate of 3 to 12° C./sec. According to the studies of the presentinventors, when martensite is produced before going through a range of440 to 480° C., the temperature range of a conventional hot-dipgalvanizing bath, coarse martensite may be formed on the cold-rolledsteel sheet to be finally obtained, thereby a low yield ratio may be notachieved. When the secondly cooling end temperature is lower than(Ms+20)° C., martensite may be generated during the secondly coolingoperation. In the meantime, when the secondly cooling end temperature ishigher than (Ms+50)° C., a cooling rate before introducing into theplating bath after the secondly cooling, that is, a thirdly cooling rateshould be controlled to be relatively high. In addition, there is a highpossibility that martensite is formed before immersing in the platingbath. When the secondary cooling rate is lower than 3° C./sec,martensite may be not formed, but it is disadvantageous in terms ofproductivity. On the other hand, when the rate exceeds 12° C./sec, theoverall speed of passing a sheet may be increased to generate problemssuch as shape warping of a sheet. For reference, the theoreticaltemperature of Ms can be obtained by the following Relationship 4:

[Relationship 4] Ms(° C.)=539−423[C]−30.4[Mn]−12.1[Cr]−17.7[Ni]−7.5[Mo]

where each of [C], [Mn], [Cr]. [Ni] and [Mo] refers to weight % of therespective elements.

Next, the secondly cooled cold-rolled steel sheet may be thirdly cooledto a temperature range of 440 to 480° C. at a rate of 4 to 8° C./sec.The above temperature range may be a temperature range of a conventionalgalvanizing bath, and this operation may be carried out to preventformation of a martensite structure before the cold-rolled steel sheetis immersed in the galvanizing bath. When the thirdly cooling rate islower than 4° C./sec, martensite may be not formed, but it isdisadvantageous in terms of productivity. On the other hand, when therate exceeds 8° C./sec, martensite may be partially formed and bainitemay be partially formed in the grains. In this case, ductility may bedeteriorated, as well as an increase in yield strength.

Next, the thirdly cooled cold-rolled steel sheet may be immersed in azinc based hot bath to obtain a steel sheet hot-dip plated with zincbased layer. In the present disclosure, a composition of the zinc basedhot bath is not particularly limited, and may be a pure galvanizing bathor an alloyed galvanizing bath containing Si, Al, Mg, or the like.

Next, the hot-dip galvanized steel sheet may be finally cooled to atemperature in a range of (Ms-100)° C. or lower at an average coolingrate of 3° C./sec or higher. When the final cooling end temperature islower than (Ms-100)° C., not only fine martensite may not be obtained,but also a defective problem regarding a plate shape may be caused.Further, when the average cooling rate is lower than 3° C./sec,martensite may be irregularly formed in the grain boundaries or in thecrystal grains, due to the excessively slow cooling rate. In addition,since a ratio of martensite formation in the crystal grains tomartensite formation in the grain boundaries is relatively low, steelhaving a relatively low yield ratio may be not manufactured.

Meanwhile, when necessary, the steel sheet hot-dip plated with zincbased layer may be subjected to an alloying heat treatment before thefinal cooling to obtain a galva-annealed steel sheet. In the presentdisclosure, conditions of the alloying heat treatment process are notparticularly limited, and may be conventional conditions. As an example,an alloying heat treatment process may be performed in a temperaturerange of 480 to 600° C.

Next, when necessary, the final cooled steel sheet plated with zincbased layer or the galva-annealed steel sheet is subjected to temperrolling to form large amounts of dislocations in ferrite disposed aroundmartensite, thereby further improving bake hardenability.

At this time, a reduction ratio is preferably 0.3 to 1.6%, morepreferably 0.5 to 1.4%. When the reduction ratio is less than 0.3%,sufficient dislocations may be not formed and it is disadvantageous fromthe viewpoint of a plate form. In particular, defects of the platedsurface may occur. On the other hand, when the reduction ratio exceeds1.6%, it is advantageous in terms of formation of dislocation, but itmay cause side effects such as occurrence of strip breakage due tofacility capability limit.

Mode for Invention

Hereinafter, the present disclosure will be described in more detail byway of examples. However, the following examples are only illustrativeof the present disclosure in more detail, and do not limit the scope ofthe present disclosure.

After preparing a steel slab having an alloy composition shown in Table1 below, a hot-dip galvanized steel sheet (GI steel sheet) or agalva-annealed steel sheet (GA steel sheet) was prepared using amanufacturing process described in Table 2 below. For reference,inventive steels 1, 2, 4 and 5 and comparative examples 1 and 2correspond to galva-annealed steel sheets in Table 1, and inventionsteels 3 and 6 correspond to hot-dip galvanized steel sheets. Meanwhile,in a preparation of each specimen, a firstly cooling end temperature wasconstantly set to be 650° C., a secondly cooling end temperature wasconstantly set to be 560° C., a thirdly cooling end temperature wasconstantly set to be 460° C., and a plating bath temperature wasconstantly set to be 480° C.

Thereafter, microstructures were observed on each of the produced platedsteel sheets, and the properties thereof were evaluated. The resultstherefrom were shown in Table 3 below.

In Table 3, fractions of microstructures and concentration ratios of Cand Mn were results from analysis of structures at the point of ¼ t of asheet thickness of the steel sheet. First, the fractions ofmicrostructures were measured by observing martensite and bainitethrough Lepera etching using an optical microscope, observing them withSEM (3,000 times), and measuring size and distribution of martensite atthree times averages through Count Point operation. Meanwhile, theconcentration ratios of C and Mn were performed by preferentiallymeasuring concentrations of C and Mn existing on the respective phasesby a CPS (Count Per Sec) method, in a line and point manner using a TEMand an EDS (Energy Dispersive Spectroscopy) analysis method, therebyquantitatively measuring the ratios. At this time, as a criterion formeasuring concentrations of C and Mn in ferrite and martensite,concentrations of C and Mn measured in a position in contact with avirtual circle having a diameter corresponding to a short axis ofmartensite were taken as an average carbon concentration in martensite,and concentrations of C and Mn measured in a ferrite in contact with avirtual circle having a diameter corresponding to a long axis ofmartensite were taken as an average carbon concentration in ferrite.

Tensile test for each specimen in Table 3 was performed in a C directionusing the JIS standard. In the meantime, the bake hardenability wasevaluated by a difference in yield strength after maintaining thespecimen at 170° C. for 20 minutes, based on the strength after a 2%pre-strain. The aging resistance was evaluated by measuring YP-El (%) atthe time of tensile test after maintaining the specimen at 100 for 2hours.

TABLE 1 Cold-Rolled Steel Sheet Composition (wt %) Classification C MnSi Cr P S N sol. Al Mo B Inventive steel 1 0.023 1.7 0.05 0.80 0.050.005 0.003 0.018 0.15 0.0006 Inventive steel 2 0.038 1.72 0.04 0.480.05 0.005 0.003 0.04 0.12 0.0009 Inventive steel 3 0.052 1.51 0.10 0.430.03 0.007 0.004 0.05 0.13 — Inventive steel 4 0.051 1.54 0.15 0.81 0.040.004 0.003 0.041 0.15 0.0021 Inventive steel 5 0.069 1.43 0.22 0.870.02 0.003 0.004 0.052 0.18 — Inventive steel 6 0.075 1.32 0.21 0.080.03 0.004 0.008 0.025 0.08 0.0012 Comparative 0.096 1.21 0.62 1.18 0.120.006 0.003 0.042 0.45 0.004 stteel 1 Comparative 0.098 1.26 0.81 1.210.12 0.007 0.005 0.05 0.38 0.0041 stteel 2

TABLE 2 Manufacturing Conditions Finish Coiling Annealing 1st 2nd 3rdFinal Reheating Rolling Temper- Cooling Temper- Cooling Cooling CoolingCooling Temperature Temperature ature Reduction ature Rate Rate RateRate Classification (° C.) (° C.) (° C.) Ratio (%) (° C.) (° C./sec) (°C./sec) (° C./sec) (° C./sec) Note Inventive 1184 882 598 48 766 2.5 4.14.5 4.5 Inventive Steel 1 example 1 1187 895 556 54 764 2.4 4.5 4.6 5.7Inventive example 2 Inventive 1183 912 465 63 777 3.4 3.4 5.1 6.2Inventive Steel 2 example 3 1183 921 472 64 779 3.6 3.5 5.5 6.3Inventive example 4 Inventive 1200 891 682 71 811 4.9 6.3 6.3 9.2Inventive Steel 3 example 5 1203 896 645 72 815 4.2 6.8 6.2 9.6Inventive example 6 Inventive 1197 935 580 75 741 5.6 9.1 7.8 5.3Comparative Steel 4 example 1 1198 942 585 79 821 5.8 10.6 7.5 7.8Inventive example 7 Inventive 1185 923 652 63 857 6.8 11.4 9.2 7.2Comparative Steel 5 example 2 1185 912 632 65 839 8.5 12.6 7.1 6.4Comparative example 3 Inventive 1209 897 682 35 841 7.5 8.5 9.2 5.2Comparative Steel 6 example 4 1205 890 647 68 835 16.5 7.8 9.5 8.9Comparative example 5 Comparative 1203 897 660 72 802 2.8 6.5 11.5 5.3Comparative steel 1 example 6 Comparative 1199 892 672 75 802 3.8 6.56.8 5.2 Comparative steel 2 example 7 1187 885 682 78 779 4.1 7.8 8.33.8 Comparative example 8

TABLE 3 Properties Microstructure YP-E1 L-BH E1 TS Classification{circle around (1)} {circle around (2)} {circle around (3)} {circlearound (4)} {circle around (5)} (%) (MPa) (%) (MPa) YR Note Inventive2.2 1.4 92.2 1.25 0.75 0 42 34 476 0.55 Inventive example 1 steel 1 1.91.7 91.4 1.2 0.69 0 41 34 468 0.56 Inventive example 2 Inventive 3.3 —90.6 1.12 0.65 0 48 36 502 0.55 Inventive example 3 steel 2 3.5 0.5 92.10.98 0.79 0 38 35 505 0.56 Inventive example 4 Inventive 4.5 — 92.3 0.890.63 0 51 35 496 0.55 Inventive example 5 steel 3 5.0 0.1 91.5 1.12 0.750 43 35 513 0.56 Inventive example 6 Inventive 0.7 3.5 90.6 1.75 0.93 028.5 26 582 0.59 Comparative example 1 steel 4 6.8 2.1 90.5 1.08 0.71 039 33 612 0.56 Inventive example 7 Inventive 4.1 0.3 77 1.08 0.63 0.4 5533 531 0.62 Comparative example 2 steel 5 6.2 1.2 78 1.55 0.83 0.3 54 28535 0.61 Comparative example 3 Inventive 1.8 0.5 93 1.72 0.92 0 28 32532 0.59 Comparative example 4 steel 6 9.8 1.5 76 1.71 0.93 0 26 26 5280.59 Comparative example 5 Comparative 4.1 3.5 77 1.67 0.92 0.3 52 26554 0.63 Comparative example 6 steel 1 Comparative 4.3 3.1 81 1.58 0.930 36 25 555 0.71 Comparative example 7 steel 2 4.2 3.3 83 1.21 0.91 0 4326 548 0.70 Comparative example 8 Wherein {circle around (1)} refers toarea ratios (%) of martensite, {circle around (2)} refers to area ratios(%) of bainite, {circle around (3)} refers to area ratios (%) offerrite, {circle around (4)} refers to (a/b) values, and {circle around(5)} refers to (d/c) values.

Referring to Table 3, in cases of Inventive Examples 1 to 7, whichsatisfy the alloy composition and manufacturing conditions proposed inthe present disclosure, tensile strengths of 450 to 650 MPa wereobtained and strengths were thus excellent, yield ratios of 0.57 or lesswere obtained and yield ratios were thus relatively low, elongations of33% or more were obtained and were thus excellent in ductility, amountsof bake hardenability (BH) of 35 MPa or more were obtained and were thusexcellent in bake hardenability, and an YP-El value of 0% was obtainedand was thus excellent in aging resistance.

On the other hand, in Comparative Example 1, since the annealingtemperature thereof was lower than the range proposed in the presentdisclosure, austenite was not sufficiently formed during the annealingoperation, and martensite was not sufficiently formed in a finalstructure. Thus, the desired ductility and bake hardenability could notbe obtained. In Comparative Example 2, the annealing temperatureexceeded the range proposed in the present disclosure. In this case,bake hardenability was secured by a formation of a martensite structure,but an aging problem was caused. Further, in Comparative Examples 3 and4, the secondly or thirdly cooling rate exceeded the range proposed inthe present disclosure. In these cases, the intended curing propertieswere not secured, or aging problems were caused. In Comparative Example5, the firstly cooling rate exceeded the range suggested in the presentdisclosure. In this case, a diffusion of carbon during cooling operationcould not sufficiently occur, and the desired bake hardenability in thepresent disclosure could not be secured. In addition, in ComparativeExamples 6 to 8, since contents of C and Cr in the steel were relativelyhigh, large amounts of bainite were formed on the whole, and elongationsthereof were relatively low.

While exemplary aspects have been shown and described above, it will beapparent to those skilled in the art that modifications and variationscould be made without departing from the scope of the present inventionas defined by the appended claims.

1. A steel sheet hot-dip plated with zinc based layer, having excellentbake hardenability and aging resistance, comprising a cold-rolled steelsheet and a zinc based plating layer formed on a surface of thecold-rolled steel sheet, wherein the cold-rolled steel sheet comprises,by weight, 0.02 to 0.08% of carbon (C), 1.3 to 2.1% of manganese (Mn),0.3% or less (excluding 0%) of silicon (Si), 1.0% or less (excluding 0%)of chromium (Cr), 0.1% or less (excluding 0%) of phosphorus (P), 0.01%or less (excluding 0%) of sulfur (S), 0.01% or less (excluding 0%) ofnitrogen (N), and 0.01 to 0.06% of acid soluble aluminum (sol.Al),comprises one or more selected from the group consisting of 0.2% or less(excluding 0%) of molybdenum (Mo) and 0.003% or less (excluding 0%) ofboron (B), and comprises a remainder of iron (Fe) and unavoidableimpurities, and comprises, by area, 90 to 99% of ferrite and 1 to 10% ofmartensite as a microstructure, wherein a ratio (a/b) of an averagecarbon concentration a in the martensite and an average carbonconcentration b in the ferrite located in a virtual circle having adiameter corresponding to a long axis of the martensite at the point of¼ t of a sheet thickness of the cold-rolled steel sheet is 1.4 or less,and wherein a ratio (d/c) of an average manganese concentration c in themartensite and an average manganese concentration d in the ferritelocated in a virtual circle having a diameter corresponding to a longaxis of the martensite at the point of ¼ t of a sheet thickness of thecold-rolled steel sheet is 0.9 or less.
 2. The steel sheet hot-dipplated with zinc based layer according to claim 1, wherein an occupancyratio (M) of martensite having an average circle equivalent diameter of5 μm or less (excluding 0 μm) present at ferrite grain boundaries(including grain boundary triple points) defined by the followingRelationship 1, in the cold-rolled steel sheet, is 90% or more:[Relationship 1] M={M _(gb)/(M _(gb)+M _(in))}×100 (Where M_(gb) refersto the number of martensite having an average circle equivalent diameterof 5 μm or less (excluding 0 μm) present at ferrite grain boundaries,and M_(in) refers to the number of martensite having an average circleequivalent diameter of 5 μm or less (excluding 0 μm) present in ferritecrystal grains)
 3. The steel sheet hot-dip plated with zinc based layeraccording to claim 1, wherein the cold-rolled steel sheet furthercomprises bainite as a microstructure, and an area ratio (B) of thebainite defined by the following Relationship 2 is 3 or less:[Relationship 2] B={A _(B) /(A _(F)+A _(M)+A _(B))}×100 (Where A_(F)refers to an area ratio of ferrite, A_(M) refers to an area ratio ofmartensite, and A_(B) refers to an area ratio of bainite)
 4. The steelsheet hot-dip plated with zinc based layer according to claim 1, whereinthe zinc based plating layer is a galva-annealed layer.
 5. The steelsheet hot-dip plated with zinc based layer according to claim 1, whereinthe steel sheet hot-dip plated with zinc based layer has the bakehardenability (BH) of 35 MPa or more.
 6. The steel sheet hot-dip platedwith zinc based layer according to claim 1, wherein the steel sheethot-dip plated with zinc based layer has a yield ratio of 0.57 or lessand an elongation of 33% or less.
 7. A method of manufacturing a steelsheet hot-dip plated with zinc based layer, having excellent bakehardenability and aging resistance, comprising: reheating a steel slabcomprising, by weight, 0.02 to 0.08% of carbon (C), 1.3 to 2.1% ofmanganese (Mn), 0.3% or less (excluding 0%) of silicon (Si), 1.0% orless (excluding 0%) of chromium (Cr), 0.1% or less (excluding 0%) ofphosphorus (P), 0.01% or less (excluding 0%) of sulfur (S), 0.01% orless (excluding 0%) of nitrogen (N), and 0.01 to 0.06% of acid solublealuminum (sol.Al), comprising one or more selected from the groupconsisting of 0.2% or less (excluding 0%) of molybdenum (Mo) and 0.003%or less (excluding 0%) of boron (B), and comprising a remainder of iron(Fe) and unavoidable impurities; hot-rolling the reheated steel slab ina single phase temperature region of austenite to obtain a hot-rolledsteel sheet; coiling the hot-rolled steel sheet; cold-rolling the coiledhot-rolled steel sheet to obtain a cold-rolled steel sheet; continuouslyannealing the cold-rolled steel sheet at a temperature in a range of 760to 850° C.; firstly cooling the continuously annealed cold-rolled steelsheet to a temperature in a range of 630 to 670° C. at an averagecooling rate of 2 to 14° C./sec; secondly cooling the firstly cooledcold-rolled steel sheet to a temperature in a range of (Ms+20) to(Ms+50)° C. at an average cooling rate of 3 to 12° C./sec; thirdlycooling the secondly cold-rolled steel sheet to a temperature in a rangeof 440 to 480° C. at a rate of 4 to 8° C./sec; immersing the thirdlycooled cold-rolled steel sheet in a zinc based hot bath to obtain asteel sheet hot-dip plated with zinc based layer; and finally coolingthe steel sheet hot-dip plated with zinc based layer to a temperature ina range of (Ms−100)° C. or lower at an average cooling rate of 3° C./secor higher.
 8. The method according to claim 7, wherein the reheatingtemperature is within a range of 1100 to 1300° C. at the time ofreheating the slab.
 9. The method according to claim 7, wherein a finishrolling temperature at the time of the hot-rolling is within a range of(Ar3+50) to 950° C.
 10. The method according to claim 7, wherein thecoiling temperature at the time of the coiling is within a range of 450to 700° C.
 11. The method according to claim 7, wherein a cold-reductionratio at the time of the cold-rolling is 40 to 80%.
 12. The methodaccording to claim 7, wherein the annealing temperature at the time ofthe continuously annealing is within a range of 770 to 810° C.
 13. Themethod according to claim 7, wherein a temperature of the zinc based hotbath is within a range of 440 to 480° C.
 14. The method according toclaim 7, further comprising subjecting the steel sheet hot-dip platedwith zinc based layer to an alloying heat treatment at a temperature ina range of 480 to 600° C., before the final cooling.
 15. The methodaccording to claim 7, further comprising temper rolling at a reductionratio of 0.3 to 1.6%, after the final cooling.